Can't set Featured Image

I’m working on a site where I modified a free wp theme. On my portfolio section I add new images from the “Featured Image”-box. This worked fine until just now. If I choose an already uploaded image as featured image it only displays on the portfolio-list site, not on the portfolio-single. If I upload the image it works fine, but if I choose that same image in a new post its not showing.

portfolio list: http://www.samayoga.info/our-teachers/

and click on any to come to portfolio-single.

I’m a newbie at this so all help is much appreciated!